[GREENFOOT-687] Scenario Information fires NPE

From the Scenario menu, select Scenario Information will open the readme file but the following NPE will be fired.

P.S. This is only happened for the first time after opening the scenario and no editor was open.

Exception in thread "JavaFX Application Thread" java.lang.NullPointerExceptionException in thread "JavaFX Application Thread" java.lang.NullPointerException at: {quote}bluej.editor.moe.MoeSyntaxDocument.getTokensForLine(MoeSyntaxDocument.java:819) at bluej.editor.moe.BlueJSyntaxView.getTokenStylesFor(BlueJSyntaxView.java:233) at bluej.editor.moe.MoeSyntaxDocument.applyPendingScopeBackgrounds(MoeSyntaxDocument.java:630) at bluej.editor.moe.MoeSyntaxDocument.fireChangedUpdate(MoeSyntaxDocument.java:553) at bluej.editor.moe.BlueJSyntaxView.lambda$setEditorPane7(BlueJSyntaxView.java:467) at bluej.utility.javafx.JavaFXUtil.lambdaaddChangeListenerPlatform$25(JavaFXUtil.java:1443) {quote}

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Issue metadata

• Issue type: Task
• Priority: Medium